Not Applicable
Not Applicable
1. Field of Invention
This invention pertains to positron emission tomography (PET) and single photon emission computed tomography (SPECT) scanners. More particularly, this invention pertains to apparatus and methods for simulating a sheet source with a line source for determining normalization coefficients for the scanner detectors.
2. Description of the Related Art
Various techniques are used for medical imaging. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are popular in radiology because of their ability to non-invasively study physiological processes and structures within the body.
Positron Emission Tomography is a nuclear imaging technique used in the medical field to assist in the diagnosis of diseases. PET allows the physician to examine the whole patient at once by producing pictures of many functions of the human body unobtainable by other imaging techniques. In this regard, PET displays images of how the body works (physiology or function) instead of simply how it looks. PET is considered the most sensitive, and exhibits the greatest quantification accuracy, of any nuclear medicine imaging instrument available at the present time. Applications requiring this sensitivity and accuracy include those in the fields of oncology, cardiology, and neurology.
In PET, short-lived positron-emitting isotopes, referred to as radiopharmaceuticals, are injected into a patient. When these radioactive drugs are administered to a patient, they distribute within the body according to the physiologic pathways associated with their stable counterparts. As the radiopharmaceutical isotopes decay in the body, they discharge positively charged particles called positrons. Upon discharge, the positrons encounter electrons, and both are annihilated. As a result of each annihilation event, gamma rays are generated in the form of a pair of diametrically opposed photons approximately 180 degrees (angular) apart. After the PET scanner detects these annihilation xe2x80x9cevent pairsxe2x80x9d over a period of time, the isotope distribution in a cross section of the body is reconstructed. These events are mapped within the patient""s body, thus allowing for the quantitative measurement of metabolic, biochemical, and functional activity in living tissue. More specifically, PET images (often in conjunction with an assumed physiologic model) are used to evaluate a variety of physiologic parameters such as glucose metabolic rate, cerebral blood flow, tissue viability, oxygen metabolism, and in vivo brain neuron activity.
Mechanically, a PET scanner consists of a bed, or gurney, and a gantry supporting the tomograph detectors. In some tomographs, the gantry is inside an enclosure having a tunnel through its center, through which the bed traverses. In other tomographs, the detectors are cantilevered over the front of the gantry. In all types of tomographs, the gantry defines a tunnel through which the patient travels. The patient, who has been treated with a radiopharmaceutical, lies on the bed and is moved longitudinally past the detectors. There are four classes of PET tomographs, based on the arrangement of the detectors. Fixed ring scanners have numerous small detectors organized in detector blocks, which are grouped into buckets, and arranged in an arc around the circumference of the gantry. A second class of PET tomographs includes fixed polygonal arrangements of panel detectors. A third class includes detectors arranged in an arc around the circumference of the gantry, with the detectors rotating about the axis of the gantry. A fourth class includes polygonal arrangements of panel detectors, with the panel detectors rotating about the axis of the gantry.
Another known tomography system is single photon emission computed tomography (SPECT). Like PET, SPECT is used to produce an image of organ functions by measuring radiation emitted from a radiopharmaceutical that is inside a patient. However, unlike PET, which detects photon pairs, SPECT detects single photons emitted by the radiopharmaceutical isotope decay. Gamma cameras are used to detect the emitted photons. These gamma cameras typically revolve about a patient, and include collimators and photon-sensitive detectors. The radiopharmaceuticals typically used include Technetium-99 and Thallium-201.
Both PET and SPECT are designed to measure the amount of radioactivity along many lines of response (LOR) that pass through the patient and are intercepted by the scanner""s detectors. Measurement errors are always present, and in many cases must be corrected by the software that processes the measurements. In particular, the response measured on each LOR is subject to an error in magnitude. Normalization coefficients represent the relationship between the measured and actual magnitude of radiation and are used to correct the magnitude errors. Normalization coefficients are determined by measuring the difference in sensitivity or efficiency of the detectors in the scanners. Normalization of scanner data is usually performed by estimating the sensitivity or efficiency of a LOR.
xe2x80x9cAn Investigation of Factors Affecting Detector and Geometric Correction in Normalization of 3-D PET Data,xe2x80x9d by Dale L. Bailey, David W. Townsend, Paul E. Kinahan, Sylke Grootoonk, and Terry Jones, IEEE Transactions on Nuclear Science, Vol. 43, No. 6, pp. 3300-07, December 1996, describes an apparatus for moving a line source to simulate a plane source for determining normalization coefficients in a PET scanner. The apparatus is an aluminum support carriage with a dc motor driven worm drive that moves the line source along the longitudinal axis of the patient tunnel.
Apparatus and methods for simulating a sheet or planar source with a line source for determining normalization coefficients for PET or SPECT scanner detectors are disclosed. According to one embodiment of the present invention, a line source, oriented parallel to the axis of the patient tunnel, is moved along a diameter of the tunnel. In another embodiment, a line source, oriented perpendicular to the patient tunnel, is moved along the axis of the patient tunnel. These two embodiments simulate a sheet source with a line source fixed to the patient bed, and the bed moving the line source within the patient tunnel of a scanner and the scanner detectors in a stationary, fixed position.
In still another embodiment, an annular sheet source is simulated with a stationary line source, oriented parallel to the axis of the tunnel and offset from the center of the tunnel, with a set of detectors rotating about the tunnel axis in a PET or SPECT scanner. Alternatively, the line source is rotated about the axis of a set of stationary detectors mounted on a fixed ring or gantry. In either case, the detectors see an annular sheet source, and a sinogram is generated. In another embodiment, a shaped attenuator surrounds the line source to ensure each detector receives equal flux levels of radiation because, with the line source offset from the center axis, the line source is positioned nearer one detector than its opposite member. The shaped attenuator increases the scattered radiation from the line source. For other embodiments, an attenuating medium is used to increase the scattered radiation.